Buckler (5e Equipment)

From D&D Wiki

A buckler is a small metal shield held in a fist grip. As it is not strapped, it can be donned or doffed as easily as drawing or stowing a weapon. Wielding a buckler increases your Armor Class by 1. You can benefit from only one shield at a time.

If you are proficient with light armor and with the rapier, then you can wield this buckler with proficiency even if you are not otherwise proficient with shields.